title: Pore waters regulate ion permeation in a calcium release-activated calcium channel.
abstract: The recent crystal structure of Orai, the pore unit of a calcium release-activated calcium (CRAC) channel, is used as the starting point for molecular dynamics and free-energy calculations designed to probe this channel's conduction properties. In free molecular dynamics simulations, cations localize preferentially at the extracellular channel entrance near the ring of Glu residues identified in the crystal structure, whereas anions localize in the basic intracellular half of the pore. To begin to understand ion permeation, the potential of mean force (PMF) was calculated for displacing a single Na(+) ion along the pore of the CRAC channel. The computed PMF indicates that the central hydrophobic region provides the major hindrance for ion diffusion along the permeation pathway, thereby illustrating the nonconducting nature of the crystal structure conformation. Strikingly, further PMF calculations demonstrate that the mutation V174A decreases the free energy barrier for conduction, rendering the channel effectively open. This seemingly dramatic effect of mutating a nonpolar residue for a smaller nonpolar residue in the pore hydrophobic region suggests an important role for the latter in conduction. Indeed, our computations show that even without significant channel-gating motions, a subtle change in the number of pore waters is sufficient to reshape the local electrostatic field and modulate the energetics of conduction, a result that rationalizes recent experimental findings. The present work suggests the activation mechanism for the wild-type CRAC channel is likely regulated by the number of pore waters and hence pore hydration governs the conductance.
authors.affiliation: Institute for Computational Molecular Science, Temple University, Philadelphia, PA 19122.
authors.forename: Hao
authors.initials: H
authors.lastname: Dong
authors.forename: Giacomo
authors.initials: G
authors.lastname: Fiorin
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Carnevale
authors.forename: Werner
authors.initials: W
authors.lastname: Treptow
authors.forename: Michael L
authors.initials: ML
authors.lastname: Klein
mesh_terms: D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004058:Diffusion; D057927:Hydrophobic and Hydrophilic Interactions; D015640:Ion Channel Gating; D017136:Ion Transport; D007477:Ions / Q000378:metabolism*; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D009154:Mutation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D055672:Static Electricity; D013816:Thermodynamics; D014867:Water / Q000378:metabolism*
chemical_list: D015220:Calcium Channels; D007477:Ions; D014867:Water; D002118:Calcium
keywords: computer simulation; store-operated calcium entry